The invention discloses a computer and an expandable power supply system. The expandable power supply system includes N interface units, a determination unit, and a voltage converting unit. N is an integer equal to or more than two. The interface units are electrically connected with at least one power supplies and switching the levels of (N−1) control signals according to conductance of the power supplies. When the interface units are electrically connected with 1st to Mth power supplies, the determination unit outputs a start signal when the determination unit receives power reply signals provided by the 1st to Mth power supplies. M is an integer, and 1≦M≦N. The voltage converting unit is enabled according to the start signal to distribute operation voltages provided by the 1st to Mth power supplies by utilizing the control signals to generate a supply voltage.